The invention relates to a superplastically formed-metallurgically bonded sandwich structure having an odd number (at least three) of core sheets with portions of the core sheets being layered and unlayered, and a method of fabricating such structures.
It has been known for many years that certain metals, such as titanium and many of its alloys, exhibit superplastic properties. Superplasticity is the capability to develop unusually high tensile elongation with reduced tendency towards necking, within a limited temperature and strain rate range. Aluminum alloys and titanium alloys exhibit good superplastic characteristics.
Diffusion bonding is a metallurgical bonding or joining of similar metallic parts which are pressed together at elevated temperatures and pressures. This causes deformation which results in intimate contact of the surfaces to be joined and subsequent diffusion of the atomic structure. A monolithic metallic piece is formed with joint strength equivalent to that of the parent metal. Diffusion bonding is accomplished entirely in the solid state.
Many of the same alloys used in superplastic forming can also be used in diffusion bonding. When the two processes are combined, the temperatures and pressures for both processes are similar so that the finished product can be formed in what is essentially a one-step operation (see U.S. Pat. No. 3,927,817 to Hamilton, et. al, which is incorporated by reference herein).
However, one disadvantage of this process involves the difficulty in making thicker sandwich structures. Such structures require considerable elongation of the superplastic core materials which may result in a finished product unable to support heavy transverse shear loads, i.e., the wider core spacing needed to accommodate the thicker core sheets may result in a weakened structure.
One reinforced core structure is shown in U.S. Pat. No. 4,217,397 by Hayase, et. al. The structure shown in FIG. 16 of that patent involves a sandwich structure having four sheets (two parallel face sheets, and two core sheets). The two core sheets are diffusion bonded together over their entire length. The core sheets are fusion welded to the face sheets. In order to make this sandwich thicker, the distance between the face nodes and the distance between the face sheets must be increased. This may result in an unstable structure when loads are applied between the face nodes. Also, the core sheets must be of a thicker gauge to support heavier loads which may make the sandwich too heavy for practical use. Another disadvantage of this structure is that the fusion welds provide poor fatigue resistance, and also make the structure difficult to manufacture.
What is required is a thicker, lightweight sandwich structure and a method of making such a structure that overcomes the disadvantages of the prior art. Such a structure would be capable of supporting large transverse shear loads.